Due to the ever more stringent legal regulations regarding exhaust emission of motor vehicles, in particular regarding emission of nitrogen oxides, recirculation of combustion exhaust to the inlet side of the internal combustion engine is state of the art in the field of internal combustion engines. The combustion gases themselves do not participate again in the combustion process in the combustion chamber of the internal combustion engine so that they constitute an inert gas that dilutes the mixture of combustion air and fuel in the combustion chamber and ensures more intimate mixing. It is thus possible to minimize the occurrence of what are termed hot spots during the combustion process, the hot spots being characterized by very high local combustion temperatures. Such very high combustion temperatures promote the formation of nitrogen oxides and must therefore be imperatively avoided.
Since the efficiency of an internal combustion engine is typically dependent on the temperature of the combustion air fed into the combustion chamber of the internal combustion engine, the combustion gases cannot be recirculated to the intake side immediately after having left the combustion chamber of the internal combustion engine. Instead, the temperature of the combustion gas must be significantly lowered. Typically, the temperatures of the combustion gases leaving the combustion chamber of the internal combustion engine are of 900° C. and more. The temperature of the combustion air fed to the combustion chamber of the internal combustion engine on the inlet side should, by contrast, not exceed 150° C. and preferably be significantly less than that. For cooling the recirculated combustion gases, it is known in the art to utilize what are termed exhaust recirculation coolers. Various constructions are known in the art in which the combustion gases to be cooled are usually circulated through exchanger tubes around the outer side of which a coolant flows, the coolant usually being the cooling water of the motor vehicle. For efficiency increase, it has been proposed in prior art to lead the combustion gases to be cooled through a bundle of exchanger tubes connected in parallel in terms of fluid flow, the coolant generally flowing around the tubes.
From the document DE 10 2004 019 554 A1 an exhaust gas recirculation system for an internal combustion engine is known which comprises an exhaust gas heat exchanger implemented as a two-part cast part. Since the very hot combustion gases are reactive due to the fact that the fuel never burns completely, the problem here is that it is technically difficult if not impossible to design the surfaces of a metallic cast part as inert surfaces comparable with a stainless steel surface.
From the document DE 10 2005 055 482 A1 an exhaust gas heat exchanger for an internal combustion engine is known that avoids the problems mentioned above by implementing the surfaces coming into touching contact with the hot combustion gases as non-corrosive steel surfaces. The heat exchanger tubes and the housing accommodating the heat exchanger tubes are configured to be separate parts that are assembled during the manufacturing process.
In the exhaust gas heat exchanger known from the document DE 10 2006 009 948 A1, the channels carrying the hot gas and the housing in which the coolant flowing around the exhaust channels flows are configured integrally in the form of a plate heat exchanger. The flow paths for the hot combustion gases as well as the flow paths for the coolant only form when individual, for example deep-drawn plates are being assembled to form a plate heat exchanger. A similar concept is pursued in the document DE 10 2006 049 106 A1.
General information regarding the technique of exhaust gas recirculation in internal combustion engines may be inferred from the document DE 100 119 54 A1 for example.
It would be desirable to produce a heat exchanger for an exhaust train of a motor vehicle that includes a bundle of separately formed exhaust gas carrying exchanger tubes exhibiting an improved Noise, Vibration, Harshness (NVH) behaviour over the prior art constructions.